The present invention is directed to a catheter for diagnosis and treatment of the heart, and more particularly to a catheter comprising a tip electrode having a recessed ring electrode mounted thereon.
Electrode catheters have been in common use in medical practice for many years. They are used to stimulate and map electrical activity in the heart and to ablate sites of aberrant electrical activity.
In use, the electrode catheter is inserted into a major vein or artery, e.g., femoral artery, and then guided into the chamber of the heart which is of concern. Once the catheter is positioned within the heart, the location of aberrant electrical activity within the heart is then located.
One location technique involves an electrophysiological mapping procedure whereby the electrical signals emanating from the conductive endocardial tissues are systematically monitored and a map is created of those signals. By analyzing that map, the physician can identify the interfering electrical pathway. A conventional method for mapping the electrical signals from conductive heart tissue is to percutaneously introduce an electrophysiology catheter (electrode catheter) having mapping electrodes mounted on its distal extremity. The catheter is maneuvered to place these electrodes in contact with or in close proximity to the endocardium. By monitoring the electrical signals at the endocardium, aberrant conductive tissue sites responsible for the arrhythmia can be pinpointed.
For mapping, it is desirable to have a relatively small mapping electrode. It has been found that smaller electrodes record more accurate and discrete electrograms. Additionally, if a bipolar mapping arrangement is used, it is desirable that the two electrodes of the mapping arrangement be in close proximity to each other and that they be similar in size to produce more accurate and useful electrograms.
Additionally, it has been found that standard bipolar mapping catheters have limited accuracy because both electrodes pick up near field electrical signals emanating from the conductive endocardial tissues due to their contact with the heart tissue, and far-field electrical signals that propagate from other regions of the heart due to their contact with the blood. The far-field signals interfere with the near-field signals, making accurate measurement of the near-field signals difficult.
Once the origination point for the arrhythmia has been located in the tissue, the physician uses an ablation procedure to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities and restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels.
A typical ablation procedure involves providing a reference electrode, generally taped to the skin of the patient. RF (radio frequency) current is applied to the tip electrode, and current flows through the media that surrounds it, i.e., blood and tissue, toward the reference electrode. The distribution of current depends on the amount of electrode surface in contact with the tissue as compared to blood, which has a higher conductivity than the tissue. Heating of the tissue occurs due to its electrical resistance. The tissue is heated sufficiently to cause cellular destruction in the cardiac tissue resulting in formation of a lesion within the cardiac tissue which is electrically non-conductive. During this process, heating of the electrode also occurs as a result of conduction from the heated tissue to the electrode itself. If the electrode temperature becomes sufficiently high, a thin transparent coating of dehydrated blood protein can form on the surface of the electrode. If the temperature continues to rise, this dehydrated layer can become progressively thicker, resulting in char and/or thrombus on the electrode surface. The creation of char and thrombus is unsafe, as the char and thrombus can be dislodged from the electrode during the procedure or during removal of the catheter after the procedure.
In clinical practice, it is desirable to reduce or eliminate the formation of char and thrombus and, for certain cardiac arrhythmias, to create larger and/or deeper lesions. One method for accomplishing this end is to monitor the temperature of the ablation electrode and to control the RF current delivered to the ablation electrode based on this temperature. If the temperature rises above a preselected value, the current is reduced until the temperature drops below this value.
Another method for determining whether char and thrombus is forming is by monitoring the impedance. Specifically, because dehydrated biological material has a higher electrical resistance than endocardial tissue, impedance to the flow of electrical energy into the tissue also increases. A significant impedance rise thus indicates the formation of char and/or thrombus. With a relatively large electrode, e.g., an 8 mm electrode, an impedance rise from char and thrombus formation may not be easily detected because the char and thrombus is formed over a relatively small percentage of the total surface area of the electrode. In contrast, if the electrically active surface area of the electrode is relatively small, char and thrombus will form over a relatively larger area of the electrode, making detection by impedance measurements easier.
The cooling effect of the blood on the electrode is dependent on the thermal properties of the ablation electrode, and in particular its surface area. Typically ablation electrodes are relatively long, most commonly at least about 4 mm and up to about 8 mm, to provide sufficient surface area for cooling. However, such electrodes are less suitable for mapping because, as discussed above, more accurate electrograms can be obtained with a smaller mapping electrode. It is desirable for electrophysiologists use the same catheter for mapping and ablation during a single procedure because, once a site is identified with a high resolution mapping electrode catheter as a target for therapy, it would be difficult to locate that site again with another catheter, particularly where the other catheter has a larger tip electrode. Accordingly, a need exists for an electrode catheter that has a relatively small surface area for enhanced mapping, but has good thermal properties for enhanced cooling during ablation.
Another disadvantage of typical ablation electrodes is that it is sometimes difficult to accurately predict lesion size because the lesion size can vary depending on the orientation of the ablation electrode. For example, typically a 7 French catheter (having an outer diameter of just over 2 mm) is provided with an ablation tip electrode at its distal end having a length ranging from about 4 mm to about 8 mm. If the ablation electrode is provided in perpendicular relation to the tissue, a relatively small surface area of the electrode is in contact with the tissue. In contrast, a relatively larger surface area would be in contact with the tissue if the ablation electrode were in a generally parallel relationship to the tissue, i.e., if the ablation electrode were positioned on its side. It is often difficult for the physician to determine the precise orientation of the tip electrode relative to the tissue because the tissue is not visible with fluoroscopy. The size of the lesion is often related to the amount surface area in contact with the tissue. As a result, there is less predictability in lesion size.
Thus, although it is desirable to have a relatively long electrode for cooling purposes, it is desirable to have a relatively small electrically active surface area for obtaining more discrete electrograms, for more accurately detecting char and thrombus formation, and for enhancing the predictability of lesion size, as well as to have a bipolar electrode arrangement that more accurately measures near-field activity.
The present invention is directed to a catheter that is particularly suitable for bipolar mapping and ablation. In one embodiment, the catheter comprises an elongated flexible body having a distal region and at least one lumen extending therethrough. A tip electrode is mounted on the distal region. The tip electrode has an exposed distal region having an outer diameter, a recessed central region having an outer surface proximal to the exposed distal region, and a proximal region having an outer diameter and an outer surface proximal to the central region. The recessed central region has an outer diameter less than the outer diameters of the exposed distal region and the proximal region. The central region and the proximal region are provided with an electrically insulating and thermally conductive layer over at least a portion of their outer surfaces. A ring electrode is mounted on the recessed central region. The ring electrode has an outer diameter less than the outer diameters of the exposed distal region and the proximal region.
With this design, the exposed region of the tip electrode is in direct contact with the heart tissue, and thus senses both the local activation energy (near-field signals) at the point of contact with the heart tissue and far field activation energy (far-field signals) received by the exposed region through the blood. However, the recessed electrode is protected from direct contact with the heart tissue, but does contact with surrounding blood. The close proximity of the recessed electrode to the exposed region enables the recessed electrode to receive approximately the same far-field signals as the exposed region. However, the recessed electrode does not pick up the local activation potential (near-field signals) that are received by the exposed region. Thus, this design permits the creation of high resolution electrograms.
In another embodiment, the catheter comprises an elongated flexible body having proximal and distal ends and at least one lumen extending therethrough. A tip section comprising a flexible tubing that is more flexible than the catheter body is mounted at the distal end of the catheter body. The flexible tubing has proximal and distal ends and at least on lumen extending therethrough. A tip electrode is mounted on the distal end of the flexible tubing. The tip electrode has an exposed distal region having an outer diameter, a recessed central region having an outer surface proximal to the exposed distal region, and a proximal region having an outer diameter and an outer surface proximal to the central region. The recessed central region has an outer diameter less than the outer diameters of the exposed distal region and the proximal region. The central region and the proximal region are provided with an electrically insulating and thermally conductive layer over at least a portion of their outer surfaces. A ring electrode is mounted on the recessed central region. The ring electrode has an outer diameter less than the outer diameters of the exposed distal region and the proximal region.